Explaining mycoinsecticide activity: poor performance of spray and bait formulations of Beauveria bassiana and Metarhizium brunneum against Mormon cricket in field cage studies

摘要

The Mormon cricket (Anabrus simplex Haldeman) (Orthoptera: Tettigoniidae) is a historic pest on millions of acres in the western US. During outbreak years, annual losses attributed to it have amounted to millions of dollars. Nonchemical controls are needed for environmentallysensitive areas - an increasing concern. Myco-insecticides are available but show an unpredictable and unexplainable lack of effect on Mormon crickets. This research sought to understand performance of Beauveria bassiana and Metarhizium brunneum under field conditions, compared to studies showing excellent performance under laboratory conditions. Our objectives were: (1) to evaluate B. bassiana GHA and M. brunneum F52 for potential use against Mormon crickets; (2) to compare spray and bait formulations of each fungus against immature and adult Mormon crickets; and (3) to understand the effect of Mormon-cricket body temperatures on the speed of efficacy by the two fungi. No mortality of 3rd-instar Mormon crickets was detected in the field 14 d after treatment with fungal baits or sprays of either B. bassiana Strain GHA or M. brunneum Strain F52. Unexpected bait aversion may have adversely affected the success of the bait treatments. Lack of spray efficacywas also unexpected, given the laboratory infectivity of both fungi for the Mormon cricket. Possible body temperatures were estimated by thermal surrogates located on the ground and in the plant canopy within field cages. Surrogate temperatures below 18°C, as well as above 30°C, were common during the study. When cumulative amount of time at temperatures optimal for fungal growth (18 to 30°C) was calculated for the duration of the post-treatment observation period, there was an insufficient number of fungal growth hours to produce mortality within the 14-d post-treatment period. Our data suggest that at least 32 to 43 d (M. brunneum) or 58 to 75 d (B. bassiana) should have been required for insect mortality to occur under the conditions of the test. In a second test, targeting seventh-instar nymphs and adults, the in-field mortality was not followed, but treated individuals were incubated both indoors and in outdoor cages. Infections by B. bassiana and M. brunneum in the indoor-incubated insects caused 69 and 100% mortality (corrected for untreated controls) respectively, with 71 and 80% mycosis in cadavers. Mormon crickets in outdoor cages succumbed to unexpected high temperatures. Our data illustrate the importance of Mormon cricket body temperatures and their effects on fungal infection. Because physical location and thermoregulation may alter the environ inside the insect, at least a month may be required after treatment for infections by B. bassiana GHA and M. brunneum F52 to cause mortality. Future studies should take this longer observational time into account. There is some potential for faster fungal growth in Mormon crickets under natural conditions, as opposed to our experimental cages, particularly in the immature stages. Mormon crickets aggregate in sheltered locations under inclement weather and at night. This behavior may conserve and actually promote body temperatures more favorable to fungal development. The potential value of this aspect of Mormon cricket behavior will require additional study.